I am remembering a novel (and I'm pretty sure it might have been a trilogy) from the late 80's to the 90's. I'm not sure the actual publishing date, that's when I came across it/them, and they were softcover. They might have been young adult, but at that time they were not marketed in the bookstore as such.
To avoid false trails, it wasn't really modern day urban fantasy or anything like Shadowrun.
What I can remember is a young adult main character (I think male) observing sidhe on horseback, kind of phasing into out reality in a forested area. The Pacific Northwest might have been a location, but I'm not positive. The setting didn't seem urban at all.
I'm getting a sense that the main character may have had some Native American heritage. My memories are really faded.
Trees were an important lore / plot point. I remember the author explaining how magic interacted with oak, ash, and elm—and that ash was some sort of super conduit.
Does this ring a bell with anybody?